1. Field of the Invention
The present invention relates to a fixing apparatus employing an electromagnetic-induction method and an image forming apparatus having the fixing apparatus.
2. Description of Related Art
Conventionally, image forming apparatuses such as copier, printer, facsimile and multifunction apparatus are known to employ a fixing apparatus that uses a so-called electromagnetic-induction heating method. When employing the fixing apparatus using the electromagnetic-induction heating method, it is possible to reduce warm-up time and power consumption compared to a fixing apparatus that employs a conventional lamp heating method using a halogen lamp or the like.
When employing the fixing apparatus using the electromagnetic-induction heating method, it is preferred to reduce thicknesses of heating and fixing rollers and thus heat capacity of the rollers, in order to shorten the time to heat the rollers. However, this effort increases heat resistance of the rollers in the axial direction, thereby causing temperature difference of the rollers in the axial direction. The temperature difference of the rollers may cause adverse effects to a fixing process, such as uneven gloss level of the printed image and hot offset. In order to manage this problem, a technology is known to reduce the temperature difference of the rollers in the axial direction in order to properly perform the fixing process.
For example, an induction coil for an induction heat-generating roller can be configured with a first induction coil and a second induction coil, the first induction coil having a heating area that corresponds to a predetermined paper size width, the second induction coil having a heating area outside of the heating area of the first induction coil. A first and a second thermometers are disposed to detect roller surface temperatures of respective first and second induction coils. At the same time, a target temperature is set for each heating area of the corresponding induction coil, and current capacity to each induction coil is separately controlled (see Related Art 1).
Further, for instance, a fixing apparatus is known which has a fixing member, a coil, and a core, the fixing member heating a toner image so as to fix the toner image to a recording medium, the coil being provided opposite to the fixing member and extending in a width direction, the core being configured to change an area opposite to the coil. Changing the area opposite to the core can adjust a heating area on the fixing member in the width direction, the area being heated by electromagnetic induction of the coil (see Related Art 2).
Furthermore, for instance, a fixing apparatus is known which has a fixing roller, an opposite roller, a fixing belt, an induction heater, and a pressure roller, the opposite roller being provided in parallel to the fixing roller and having nonmagnetic material, the fixing belt being an endless belt wound around the fixing roller and the opposite roller, the induction heater performing electromagnetic-induction heating on the fixing belt, the pressure roller pressing the fixing roller via the fixing belt. The fixing belt has a layer structure that includes a base material, an elastic layer, and a release layer in order from a lower side. The base material is formed of a material dispersed with magnetic shunt alloy, whose Curie temperature is set lower than a hot offset temperature of toner (see Related Art 3).
[Related Art 1] Japanese Patent Laid-open Publication No. 2002-23557
[Related Art 2] Japanese Patent Laid-open Publication No. 2006-30885
[Related Art 3] Japanese Patent Laid-open Publication No. 2006-11217
When the above-described fixing apparatuses continuously process (feed) recording paper having a predetermined width, a temperature difference occurs on the rollers between portions corresponding to paper feeding and non-paper feeding areas of the recording paper. When recording paper having a wider width is fed thereafter, a gloss level may vary between the paper feeding and non-paper feeding areas because of a fixing temperature difference. It is difficult, however, to properly solve such a problem with the above-described conventional technologies.
More specifically, the conventional technology described in Related Art 1 above can evenly distribute the fixing temperature in the axis direction, when feeding recording paper having a predetermined paper width corresponding to the heating area of the first induction coil. However, the technology cannot handle recording paper having a different width, thus causing a problem of an uneven fixing temperature. In this case, the induction coil may be configured to include an increased number of coils so as to deal with a variety of recording paper widths (i.e., the induction coil is more divided), which, however, causes a problem that complicates the structure of the fixing apparatus.
Further, the conventional technology described in Related Art 2 above requires a mechanism (a motor or the like) for rotating the core in order to change the area opposite to the coil, thus complicating the structure of the fixing apparatus. Controlling the temperature in accordance with a plurality of recording paper widths, in particular, requires setting of a temperature sensor for each of the paper widths, thus increasing cost and complicating power control.
In the conventional technology described in Related Art 3 above, the non-paper feeding area may be maintained at a temperature lower than a hot offset temperature in a predetermined fixing job. When the temperature difference occurs between the paper feeding and non-paper feeding areas at a predetermined level or more, however, the gloss level varies when recording paper having a wider width is fed in a subsequent fixing job. In order to reduce such a temperature difference, the Curie temperature may be set around the fixing temperature. In this case, however, heating efficiency declines near the fixing temperature, thus causing a problem that extends warm-up time.